Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM)-type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements, sometimes referred to as the gas diffusion media components, that: (1) serve as current collectors for the anode and cathode; (2) contain appropriate openings therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts; (3) remove product water vapor or liquid water from electrode to flow field channels; (4) are thermally conductive for heat rejection; and (5) have mechanical strength. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (e.g., a stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the MEA described earlier, and each such MEA provides its increment of voltage.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
Examples of technology related to PEM and other related types of fuel cell systems can be found with reference to commonly-assigned U.S. Pat. No. 3,985,578 to Witherspoon et al.; U.S. Pat. No. 5,272,017 to Swathirajan et al.; U.S. Pat. No. 5,624,769 to Li et al.; U.S. Pat. No. 5,776,624 to Neutzler; U.S. Pat. No. 6,103,409 to DiPierno Bosco et al.; U.S. Pat. No. 6,277,513 to Swathirajan et al.; U.S. Pat. No. 6,350,539 to Woods, III et al.; U.S. Pat. No. 6,372,376 to Fronk et al.; U.S. Pat. No. 6,376,111 to Mathias et al.; U.S. Pat. No. 6,521,381 to Vyas et al.; U.S. Pat. No. 6,524,736 to Sompalli et al.; U.S. Pat. No. 6,528,191 to Senner; U.S. Pat. No. 6,566,004 to Fly et al.; U.S. Pat. No. 6,630,260 to Forte et al.; U.S. Pat. No. 6,663,994 to Fly et al.; U.S. Pat. No. 6,740,433 to Senner; U.S. Pat. No. 6,777,120 to Nelson et al.; U.S. Pat. No. 6,793,544 to Brady et al.; U.S. Pat. No. 6,794,068 to Rapaport et al.; U.S. Pat. No. 6,811,918 to Blunk et al.; U.S. Pat. No. 6,824,909 to Mathias et al.; U.S. Patent Application Publication Nos. 2004/0229087 to Senner et al.; 2005/0026012 to O'Hara; 2005/0026018 to O'Hara et al.; and 2005/0026523 to O'Hara et al., the entire specifications of all of which are expressly incorporated herein by reference.
FIGS. 1 and 2 illustrate typical uncontrolled overlap edge design which typically fails in the edge regions, e.g., where the edge portion 10 of the anode catalyst layer 12 abuts against the edge portion 14 of the subgasket layer 16 and the edge portion 18 of the adhesive layer 20, and/or where the edge portion 22 of the cathode catalyst layer 24 abuts against the edge portion 26 of the other subgasket layer 28 and the edge portion 30 of the other adhesive layer 32. It should be noted that the anode catalyst layer 12 and the cathode catalyst layer 24 sandwich the membrane layer 34 in an overlapping, staggered or offset configuration, as shown in FIGS. 1 and 2.
This uncontrolled overlap configuration yields an anode>cathode configuration at some MEA edge locations and a cathode>anode configuration at some other MEA edge locations typically due to manufacturing tolerances during the application of the anode and cathode subgaskets to the membrane and during the electrode decal transfer process. Limitations of these particular designs include a potential gap G between the edges of the respective catalyst layers and the edges of the respective subgaskets, as shown in FIGS. 3 and 4. A gap allows direct gas access, e.g., either O2 or H2, to the membrane surface thereby enhancing the formation of *OH and *HO2 radical species from the crossover gasses, thus leading to accelerated local chemical degradation of the polymer membrane. If the electrode is larger than the window created by the subgasket, the catalyst layer may form “tents” T over the edge of the subgasket during the catalyst application process to the membrane (typically known as decal transfer), as shown in FIGS. 5 and 6. This tenting typically leads to a catalyst crack at the edge of the subgasket and subsequent direct gas access to the membrane surface and accelerated local chemical degradation. In addition, the tenting phenomenon typically leads to some length of membrane surface, whose length is on the order of the thickness of the subgasket, with no catalyst attached to the membrane surface.
Various attempts have been made to overcome this problem. For example, some manufacturers have used hot pressure to attach the subgaskets to the ionomer membranes. In addition to the heat and pressure required in the technique, an additional disadvantage is that the catalyst layers are added after the subgasket. This prevents the subgasket from being attached over the catalyst layer. The subgasket can be attached under the catalyst in this method, but this will often lead to catalyst cracking and delaminating at the subgasket edge. This cracking results in a fuzzy catalyst edge at the subgasket edge. In order to offset the anode and cathode catalyst edges, this method requires additional space to account for the uncertainty in the catalyst edge caused by the cracking. Additionally, this method could leave gaps of exposed ionomer membrane between the catalyst and subgasket edges.
Other attempts use an approach where the subgasket is placed on top of the membrane. Then this three layer structure is sandwiched between two pieces of catalyst coated diffusion media. The entire assembly is then hot pressed past the glass transition point of the ionomer to form the MEA. While this approach is fairly robust in terms of controlling catalyst edges it has several disadvantages. First, the heat and pressure required to get the bond can cause the ionomer membrane to flow, which can lead to thinning under the subgaskets. Second, the heat/cool cycles can induce thermal stresses in the part. Failures can then occur at the subgasket edge. Third, because the entire MEA, to include the gas diffusion medium (GDM), is assembled in one hot press step, it is very difficult to then inspect the catalyst edge positions.
Accordingly, there exists a need for new and improved MEA configurations wherein the edge architecture in proximity to the subgasket and catalyst coated membrane layers provide enhanced membrane durability.